The big 2500 hp centrecabs that Lima built for the PRR have come up in the course of a discussion on the EMD forum (string: "EMD field loop dynamic brake control")

More information would be nice.Background: The Lima transfer units came in at least two variants. One variant had dynamic brakes. We'd like to know what sort of control (field loop or potential wire) the dynamic brakes had on the units with them.

Preliminary questions.---PRR had two subclasses for the Limas, LS25 and LS25m. I think the db-equipped ones were LS25m. Q. Did all LS25m have dynamic brakes?Q. Which units were which? I think I've found a source that says the LS25 and LS25m were in two different number blocks, but confirmation would be nice.

(Kirkland's book on F-M and Lima diesel locomotives says that two units from the PRR's second order of Lima units had db, but doesn't say anything about the first order.)

But the main question is about the control system: the discussion on the EMD forum centres on the history of the different systems used by several locomotive builders.

Pennsylvania Railroad received two orders of Lima transfer locomotives, 11 each in 1950 and 1951.In 1950 they got LS25 5671-5681. (These had dual controls (centre cab with long hoods: configuration would have reminded a PRR official of their GG-1 electrics, so having one control station facing in each direction would have been obvious.)

In 1951 they got two more LS25, 5682 and 5683, and nine LS25m, 8943-8951. (From the wording in Kirkland's book, I think all of these had inductive train phone aerials ("handrails") along the roof.) Of these, two -- 8948 and 8949 -- had dynamic brakes and "hump control" (special control that allowed operation at a continuous low speed while pushing cars up the hump).

(I find it a bit surprising that the same units got hump control and dynamic brakes. Dynamics tend to be good at slowing down from a high speed, but not very effective in the lowest speed ranges. I would have expected d.b. on a unit meant to go out and pull trains on a hilly main line, not on one with a control system suggesting it was intended as a dedicated classification yard "specialist." But I'm not an expert. Maybe Lima and Westinghouse came up with something like what are called "extended range" d.b. that were effective at lower speeds than most early dynamics?)

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Source: an online PRR roster at broadway.pennsyrr.com (Kirkland's Fairbanks-Morse and Lima book confirms that 8948 and 8949 were the units from the second order with d.b., but doesn't mention hum control.)

The 8900's were the LS25m units which all had MU, trainphone, and cab signals. Of this block only 8948 and 8949 had dynamic braking. Photos in Staufer's book PENNSY POWER II clearly show that the dynamic brake equipped units used 27 point jumpers and field loop connectors. I do not see hump control here in my references but can't rule it out.

Lima-Hamilton Operators Manual Form DTL in my collection, which covers the PRR LS25 units is clear in showing that these units had none of the range of options that the LS25m units had.

All we are looking at here is the same basic locomotive model with or without a range of optional items. The second order for transfer units from PRR consisted of units in both classes - LS25 and LS25m and these were featured as given above. This was as you know completely normal for the PRR, which was ordering units for specific assignments and not block ordering large batches of identical units -- yet.

Lima-Hamilton units with Westinghouse control equipment (always electric throttle) and dynamic brake will use the selector lever to control dynamic brake effort, operationally like an ALCO-GE or early EMD even if the controller itself looks completely different from those other makes. Most of the Lima-Hamilton units simply do not have a selector lever at all - it isn't even there.